1. Technical Field of the Invention
The invention relates generally to communication systems; and, more particularly, it relates to communication systems that operate under attenuated transmission conditions.
2. Description of the Related Art
The problems presented by attenuation of transmitted signals within communication systems have existed for quite some time. In many different types of communication systems, there may be an undesirable attenuation of a signal when being transmitted from a transmitter to a receiver through the infrastructure of the communication system. That is to say, a transmitter may experience a large attenuation of its transmitted signals when they travel to the receiver via the communication system.
In one type of communication system, a cable modem communication system, this path may be viewed as being an upstream or reverse path between a Cable Modem (CM) and a Cable Modem Termination System (CMTS), and/or the forward path of communication from the CMTS to any one or more CMs within the cable modem communication system.
In addition, within many communication system networks, there are a multitude of transmitter-to-receiver paths between the various devices, and there is a large variety of degrees of attenuation among all of those various paths. Some paths may have large attenuation, and some may not have so large an attenuation; there is a continuum of possible degrees of attenuation throughout the various paths within the communication system.
Referring back to the cable modem communication system (which is sometimes referred to as a cable plant), the Data Over Cable Service Interface Specification (DOCSIS) will typically govern the transmission and receipt of signals throughout the cable modem communication system. In this situation, some cable modems (CMs) may have greater attenuation than others over the transmission paths from their respective CM output to the cable headend (e.g., the CMTS contained therein). As a further example, in an apartment complex, there may be long runs of cable, including one or more splitters, connecting the various apartment units. Thus, the cabling itself from the CM may itself even introduce a large attenuation even before that particular CM cabling, within the apartment building, is attached to the rest of the cable plant.
These same effects may also be present within wireless communication systems. For example, within a wireless transmission path, where path length differences between the various devices within the system may vary greatly, with some transmitter-receivers being located relatively close and perhaps within a line-of-sight of a wireless termination system, while other transmitter-receivers may be located at a great distance from the wireless termination system and perhaps have an obscured line-of-sight and/or destructively interfering multipath.
While there are some prior art approaches to deal with the problems presented by undesirable attenuation of signals as they are transmitted through the communication system, these prior art approaches fail to address this large attenuation within the transmission path without also degrading the efficient operation for the full set of transmitters operating into a given receiver. For example, in the cable modem communication system context, these prior art approaches will themselves oftentimes introduce degradation of some, if not all, of the CMs as they transmit signals to the CMTS. In addition, these prior art approaches will typically significantly increase the complexity of the communication system's components. This increase in the complexity of the communication system's components, provided by the prior art approaches, is typically found in increases to the complexity of the Media Access Control (MAC) (sometimes referred to as the Medium Access Control) and Physical (PHY) layer components of the communication system.
One prior art means for satisfying the problem of one (or several) of the many transmitter-receiver links suffering excessive attenuation (or path loss) is to employ a receiver having certain flexibility in its operating characteristics. Such a flexible receiver would be capable of operating at a multitude of SNRs (Signal to Noise Ratios) in the network environment. The flexible receiver quickly adjusts from high SNR reception to low SNR reception and/or vice-versa, and it would utilize a MAC layer which efficiently manipulates and allocates access to the network while factoring in the variation throughput which necessarily accompanies the variety of SNRs across the various links within the communication system. These system level concepts have been proposed for this problem already in the prior art, especially in the wireless environment, under the moniker of multi-channel multipoint distribution service (MMDS) Adaptive Modulation approach. However, a major drawback of many such Adaptive Modulation approaches is the typically immense complexity associated therewith, especially, but not solely, when resolving the MAC layer issues.
In addition, within many communication systems, there is a requirement that all transmitters be constrained to use the same modulation parameters. This may be because the receiver is limited to receiving signals using that common set of modulation parameters or characteristics. As mentioned above, the prior art approach of providing such rapidly changing receiver flexibility at the PHY layer and at the MAC layer is not without a significant increase in complexity. These parameters may include the modulation order (QPSK (Quadrature Phase Shift Keying), 16 QAM (Quadrature Amplitude Modulation), 64 QAM, etc.), the FEC (Forward Error Correction) parameters (RS (Reed Solomon) codeword length N and number of correctable bytes T), and other parameters as well.
In a communication system lacking such an extremely sophisticated and flexible PHY and MAC layer, if one of the transmitters is disadvantaged and unable to communicate using the particular modulation order at hand, such as 64 QAM, then this would cause a reduction of the modulation order on the entire channel to a lower order prescribed modulation that all of the transmitters can accommodate. For example, this could involve reducing the modulation order from 64 QAM to 32 QAM, in one instance, or to an even lower modulation order as dictated by the highly attenuated and problematic transmitter-receiver path. Therefore, in doing so, all of the transmitters need to be reduced to the lower order modulation; it would require reducing all of the transmitters to 32 QAM in this example. This would undesirably reduce the raw throughput of the communication channel (bits per second) by a ratio of 5/6. Clearly, there are situations where the reduction of modulation order may be even more significant and the throughput of the communication channel would be even more affected.
Another problem that often arises in such communication systems is an upper limit on the power that a particular transmitter is capable of using, or is permitted to use, to transmit its information. Such an upper power limit may be imposed by the capabilities of economically implemented transmit amplifiers which are allocated to have a certain maximum transmit power given a spurious fidelity requirement that must be met. In addition, the regulatory agencies (e.g., the Federal Communications Commission (FCC)) may also impose a limit on transmit power to prevent interference with other services operating within other frequency spectra. In addition, in some systems, there is a nonlinear element in the communication channel which limits the power that can be passed through the communication channel. In the case of a cable modem communication system, one potential source of nonlinearity may be an upstream laser which will clips signals above a certain maximum power level.
One prior art approach that seeks to deal with these deficiencies is to increase the transmitted power of a transmitter up to a certain point so as to overcome the high attenuation of its transmitter-receiver transmission path. However, because of the inherent limitations of the device, the transmitter can not increase its power beyond an upper limit point, as described above. Again, this upper transmitted power limit could be due to standards, wherein the limitations are attempting to allow coexistence with other communications networks or broadcasts, such as wireless systems. Alternatively, the upper transmitted power limit may be caused by agreed-upon practical or cost-effective limits (as in DOCSIS), or they could be a combination of these factors. This power limitation, regardless of which source introduces it, inherently presents a limit by which this prior art approach can employ the increasing of transmitted power to address this problem.
Yet another problem that arises in such communication system is a problem associated with the multipoint-to-point connectivity within communication systems. A transmitter may need to enter the network (e.g., range and register) before it can communicate in a normal manner within the communication system. The attenuated transmission conditions may simply make prior art approaches to perform this ranging and registering impossible, given the oftentimes relatively low SNR on the communication channel of interest on which the ranging and registering is to be performed.
Also along these lines of a communication system having a communication channel that is extremely attenuated, in many multipoint-to-point communication systems, a headend receiver (e.g., a CMTS of a cable headend in a cable modem communication system) must adjust the transmission parameters of the transmitters (e.g., the CMs in a cable modem communication system) based on transmissions (such as ranging bursts) from the transmitters to the receiver. That is, the transmitter must send a ranging burst to the receiver, and the receiver must make measurements on the ranging burst and determine adjustments, if any, to one or more of the transmitter's operational parameters. These transmitter operational parameters may include timing offset, frequency offset, power, equalizer coefficients, among other parameters. However, in an attenuated channel, the ranging burst itself is likely to have a significantly reduced SNR upon arrival at the receiver. This will again make the ranging and registering of the transmitter challenging. Even if the ranging and registering of the transmitter may be performed, it is likely to be made with significant error given the significantly reduced SNR of the ranging burst upon arrival at the receiver.
Therefore, there does not presently exist, in the art, a means by which a transmitter can overcome a severe attenuation in its transmission path to the receiver and thereby maintain reliable operation at the receiver. As such, no prior art solution is able to address the even more complicated situation that arises within multipoint-to-point communication systems having numerous reflections, additional paths, etc. contained throughout the communication system.
In addition, the prior art does not presently provide a solution by which a transmitter can overcome a severe attenuation in its path to the receiver and still maintain a desired SNR at the receiver. The prior art also presents no solution by which a transmitter can increase the SNR at the receiver without increasing its transmitted signal power beyond the certain/predetermined limit as described above.
There also does not presently exist, in the art, a means by which a transmitter can reduce its own throughput while retaining its assigned modulation parameters, and hence not require the other transmitters on the communication channel to reduce their throughput as well.